1. Field of the Invention
The present invention relates to turbomachincs having a speed reducer and more particularly, to turboprop engines and high-power bypass turbojet engines, such as are used in civil aviation.
2. Discussion of Background
To improve performance while reducing noise and fuel consumption, turbomachines such as turboprop engines must have slow-running large-diameter multiple blade propellers. However, the power transmitted to a slow-running propeller from a fast-running gas turbine through a mechanical speed reducer produces considerable heat as a result of mechanical losses--i.e., due to friction. Of course, this heat must be dissipated or removed efficiently, otherwise the mechanical parts of the reducer, such as the gears and bearings, will soon be damaged or the qualities of the reducer lubricant will be impaired, leading to decreased engine efficiency. Even with an efficiency of close to or even exceeding 99%, the speed reducer of a turbomachine with a mechanical power equivalent to 10,000 kW, evolves a heat equivalent of 100 kW due to mechanical losses.
It is well known in the art to remove this kind of heat--i.e., to dissipate this kind of thermal power --by means of a closed-circuit pump or thermosyphonic circulation of the reducer lubricant through a radiator, such as an oil radiator or an oil-air exchanger.
FIG. 1 shows a schematic view of one possible example of this known form of cooling. In FIG. 1 the propeller 1 of a turboprop engine is driven by a gas generator 2 through a speed reducer 3. The speed reducer 3 is cooled by circulation of its lubricant through a cooling circuit 4 including a radiator 6 disposed in the bottom scoop 5 of the engine. The radiator 6 may also be disposed below the aircraft wing 20 or even laterally with respect to the aircraft wing 20.
In the example shown in FIG. 1, the air moving through the radiator 6 removes the heat to outside the bottom scoop 5, and the cooled lubricant returns to the speed reducer 3 through the cooling circuit 4. If required, a flap (not shown) can be provided at the entry or exit of the scoop 5 to control the rate of air flow through the radiator 6 in order to stabilize the temperature of the lubricant, since at high speed the radiator 6 is oversized, as compared with low speeds. This kind of radiator 6, particularly if it is an air-oil heat exchanger, must be large if it is to remove substantial amounts of heat. This is the case, inter alia, during low speed flight, during prolonged parking with the engine idling, and also during runway taxying. This kind of equipment is therefore heavy and bulky, increases engine drag, requires a large quantity of oil, and is very vulnerable to the intake of birds.
However, this latter disadvantage does not occur with a cooling system such as that described in French patent application No. FR-A-2 742,479 (hereinafter "FR'479"). In FR '479, the boundary layer of the air flow over the engine nacelle, particularly in the zone where this boundary flow becomes turbulent as aircraft speed increases, is aspirated via holes in the cowling so that air is drawn into a collector in which a long heat exchanger, which forms a part of the cooling system, is disposed. However, since the rate of air flow through the holes is low this cooling system needs a very long, and therefore very bulky, heat exchanger.